1. Field of the Invention
The present invention relates to an initial access setting information generating apparatus, an initial access setting information generating method, an initial access setting information generating program, and a base station apparatus, so as to generate initial access setting information used in an initial access procedure performed between a base station and a communication terminal.
Priority is claimed on Japanese Patent Application No. 2010-216287, filed Sep. 28, 2010, the contents of which are incorporated herein by reference.
2. Description of the Related Art
In an LTE (long term evolution) specification, when a communication terminal is powered on, or handover (switching of the base station) occurs, an initial access procedure (called “random access procedure”) is performed between a communication terminal and a base station (after handover (if handover is performed)) so as to establish the communication between the communication terminal and the base station. Additionally, in LTE, a contention based (i.e., contention type) random access procedure and a non-contention based (i.e., non-contention type) random access procedure are defined (see, for example, Non-Patent Document 1).
In the contention based random access procedure, the communication terminal first starts the operation. For example, a communication terminal, which has been powered on or whose state is switched from an idle state to a connected state, starts the contention based random access procedure by sending a “random access preamble” (preamble information) to a base station.
In the non-contention based random access procedure, the base station first starts the operation. For example, if handover occurs between base stations, the new base station for the handover from the previous base station generates a specific signal for the handover, and communicates the signal to the previous base station. The previous base station then sends a random access preamble assignment to a communication terminal as a target for the relevant communication.
That is, the new base station for the handover starts the non-contention based random access procedure via the previous base station. In the non-contention based random access procedure, the communication terminal which received the random access preamble assignment sends a random access preamble (preamble information) to the new base station.
The transmission (electric) power of the random access preamble sent from a communication terminal in the contention based or non-contention based random access procedure is determined based on initial access setting information sent from the relevant base station to the communication terminal. In accordance with Non-Patent Document 2 (a specification for LTE), transmission power (value) PRACH of the random access preamble from a communication terminal is represented by Formula (1):
                              P          RACH                =                  min          ⁡                      (                                                                                                      P                      CMAX                                        ,                                                                  P                        PIRTP                                            +                      PL                      +                                                                                                                                                              (                                              preambleTransmissionCounter                        -                        1                                            )                                        ×                    powerRampingStep                                                                        )                                              (        1        )            Where PCMAX indicates a maximum transmission power which is specifically assigned to the communication terminal; PPIRTP is an expected reception power (called “preamble initial received target power”) at the base station; PL (path loss) is a path loss value measured by the communication terminal; “preambleTransmissionCounter” indicates how many times the random access preamble has been sent; and “powerRampingStep” indicates an increase when the random access preamble is retransmitted.
In Formula (1), the expected reception power PPIRTP and the power increase powerRampingStep are information included in the initial access setting information sent from the base station to the communication terminal.
The random access preamble sent from the communication terminal may receive interference from a base station adjacent to the target base station. Therefore, it is desirable for the communication terminal to set the transmission power in accordance with isolation control information a variation in the state (e.g., wireless environment or the number of accesses from communication terminals) of such an adjacent base station.
Conventionally, a technique of adjusting the expected reception power PPIRTP based on statistical information measured by the base station has been examined (see, for example, Non-Patent Document 3). Non-Patent Document 3 discloses that PPIRTP is determined so as to obtain a constant rate of random access procedures in which transmission and reception of the random access preamble succeeded at the first time to all random access procedures, and the determined PPIRTP is contained in the initial access setting information.
Additionally, in LTE, the base station informs the communication terminal of the initial access setting information by including an amount of wireless resources for PRACHs (physical random access channels) in the initial access setting information, where the amount of wireless resources for PRACHs can be independently set at each base station. Since the amount of wireless resources for PRACHs may press the wireless resources of upward data channels, it is desirable for the base station to adjust the amount of wireless resources for PRACHs so as to prevent the random access preambles sent from the communication terminals from colliding with each other at the relevant base station.    Non-Patent Document 1: 3GPP TS 36.300 v9.3.0 2010-03, pp. 55-58    Non-Patent Document 2: 3GPP TS 36.213 v9.1.0 2010-03, pp. 16-17    Non-Patent Document 3: M. Amirijoo, P. Frenger, F. Gunnarsson, J. Moe and K. Zetterberg, “On self-optimization of the random access procedure in 3G Long Term Evolution”, IFIP/IEEE International Symposium on Integrated Network Management-Workshops, 2009. IM '09, pp. 177-184, June, 2009
However, the technique for adjusting the expected reception power PPIRTP disclosed in Non-Patent Document 3 is not a measure performed by the base station (after the communication terminal retransmits the random access preamble) in accordance with a cause of the retransmission. The cause for the random access preamble retransmission may be degradation in the communication quality due to relatively low reception power or relatively large interference, or code collision. In the above-described conventional adjustment technique, when the random access preamble is retransmitted due to code collision, the base station may increase the expected reception power PPIRTP in spite that the amount of resources for the random access preamble should be increased. If such inappropriate initial access setting information is used, not only an object to reduce the number of random access preamble retransmissions (i.e., the number which indicates how many times the retransmission was executed) is not achieved, but also the interference to an adjacent base station or the consumption power of the relevant communication terminal may increase.
Similarly, when the random access preamble is retransmitted due to a decrease in the reception power or an increase in the interference power, the base station may increase the amount of resources for the random access preamble in spite that the expected reception power PPIRTP should be increased. If such inappropriate initial access setting information is used, not only is an object to reduce the number of random access preamble retransmissions not achieved, but also the efficiency of using the relevant system may be reduced by pressing the data channels.